Abstract
Abstract. Idealized climate change simulations are used as benchmark experiments to facilitate the comparison of ensembles of climate models. In the fifth phase of the Coupled Model Intercomparison Project (CMIP5), the 1 % per yearly compounded change in atmospheric CO2 concentration experiment was used to compare Earth system models with full representations of the global carbon cycle in the Coupled Climate–Carbon Cycle Model Intercomparison Project (C4MIP). However, this “1 % experiment” was never intended for such a purpose and implies a rise in atmospheric CO2 concentration at double the rate of the instrumental record. Here, we examine this choice by using an intermediate complexity climate model to compare the 1 % experiment to an idealized CO2 pathway derived from a logistic function. The comparison shows three key differences in model output when forcing the model with the logistic experiment. (1) The model forced with the logistic experiment exhibits a transition of the land biosphere from a carbon sink to a carbon source, a feature absent when forcing the model with the 1 % experiment. (2) The ocean uptake of carbon comes to dominate the carbon cycle as emissions decline, a feature that cannot be captured when forcing a model with the 1 % experiment, as emissions always increase in that experiment. (3) The permafrost carbon feedback to climate change under the 1 % experiment forcing is less than half the strength of the feedback seen under logistic experiment forcing. Using the logistic experiment also allows smooth transition to zero or negative emissions states, allowing these states to be examined without sharp discontinuities in CO2 emissions. The protocol for the CMIP6 iteration of C4MIP again sets the 1 % experiment as the benchmark experiment for model intercomparison; however, clever use of the Tier 2 experiments may alleviate some of the limitations outlined here. Given the limitations of the 1 % experiment as the benchmark experiment for carbon cycle intercomparisons, adding a logistic or similar idealized experiment to the protocol of the CMIP7 iteration of C4MIP is recommended.
Highlights
Idealized climate change experiments are used as common framework to compare the output of ensembles of climate models (Houghton et al, 1996)
Two of the idealized experiments outlined by the Coupled Model Intercomparison Project (CMIP) prescribe changes only in atmospheric CO2 concentration: the 4× CO2 experiment and the 1 % per year compounded increase in atmospheric CO2 experiment (Meehl et al, 2007; Taylor et al, 2012; Eyring et al, 2016)
Idealized scenarios have a long history in the climate modelling community having been used for benchmark experiments and as a means of model intercomparison since the Second Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (Houghton et al, 1996)
Summary
Idealized climate change experiments are used as common framework to compare the output of ensembles of climate models (Houghton et al, 1996). Two of the idealized experiments outlined by the Coupled Model Intercomparison Project (CMIP) prescribe changes only in atmospheric CO2 concentration: the 4× CO2 experiment and the 1 % per year compounded increase in atmospheric CO2 experiment (hereafter referred to as the 1 % experiment) (Meehl et al, 2007; Taylor et al, 2012; Eyring et al, 2016) Using either of these experiments allows for the study of the effect of CO2 on the Earth system without having to account for the confounding effects of land use change, non-CO2 greenhouse gases, and aerosols. The 1 % experiment prescribes a rise in atmospheric CO2 concentration from pre-industrial concentration at 1 % a year compounded, resulting in a doubling of CO2 concentration at year 70 of the simulation, and a quadrupling of CO2
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
